Like most sixteen year-old boys, Gene and Finny and their friends struggle to define their identities. World War II complicates their otherwise typical teenage identity crises and forces them to define themselves first and foremost in relation to the war. Different boys do this in different ways. Leper decides to enlist, even though military life contrasts sharply with his gentle, nature loving instincts. Brinker Hadley assumes an air of bravado. Finny denies the war exists at all. In each case, the boys are trying to define themselves against something in order to be men.
Gene goes through the same identity crisis, but his crisis resolves not around war, but Finny. Gene's admiration for and jealousy of his friend is so great that he literally loses himself in Finny. At one point he secretly dresses up in Finny's clothes. At another time when Finny is injured, he feels that he doesn't exist. Gene's personal identity is so wrapped up in Finny that in order to become an individual with his own identity, he has to destroy Finny. Through this tragedy, A Separate Peace makes the case that in the effort to define themselves as they grow into adults, people create false enemies out of true friends.